This disclosure relates to method for the production of polycarbonates. Aromatic polycarbonates are well-known thermoplastic materials, which are in wide use as engineering plastics in many fields because of their excellent mechanical properties, for example high impact resistance, heat resistance, flexibility, and transparency.
Conventional industrial plants synthesize polycarbonates by interfacial polymerization, i.e., by mixing together an aqueous solution of an aromatic dihydroxy compound with an organic solution of a carbonate precursor such as phosgene. Upon mixing the immiscible organic and aqueous phases, the dihydroxy compound reacts with the carbonate precursor at the phase interface. Typically, a phase transfer catalyst, such as a tertiary amine, is added to the aqueous phase to enhance the reaction.
The interfacial method for making polycarbonate has several inherent disadvantages. Plant capacity and product quality are often limited by the need to separate the organic and aqueous phases, which can be difficult due to the high viscosity of the organic phase (which contains the dissolved high molecular weight polycarbonate products). Isolation of the polycarbonate is also problematic, as complete removal of the organic solvent from the thick, viscous polycarbonate product requires expensive techniques such as solvent interchange (e.g., with monochlorobenzene), evaporative extrusion or steam precipitation followed by several drying steps. Accordingly, there remains a need in the art for more efficient and effective methods for the large-scale production of polycarbonates involving interfacial polymerization.